1. Field of the Invention
This invention relates cholesteric liquid crystal devices. More importantly, the present invention relates to a single layer multi-state ultra-fast cholesteric liquid crystal device for switching.
2. Description of the Related Art
An electro-opticallyactive cholesteric liquid crystal (CLC) known in the prior art can possibly exhibit some of the following optical states depending upon the external conditions:
(1) The CLC can be operated in a narrow band reflective state associated with the spontaneous planar alignment texture of the CLC molecules with a constant helical pitch as disclosed by, P. G. de Gennes and J. Prost, xe2x80x9cThe Physics of Liquid Crystalsxe2x80x9d, (Oxford University Press, New York, 1993, 2nd Edition); or
(2) The CLC can be operated in a broadband reflective state associated with the disturbed planar alignment texture of the CLC molecules with a pitch gradient as disclosed by, xe2x80x9cSingle Layer Reflective Polarizers with Electrically Controlled Spectrumxe2x80x9d, Jianfeng Li, B. Fan, and Le Li, SID""97, 1999; or
(3) The CLC can be operated in a focal conic state that optically scatters light in the visible region; or
(4) The CLC can be operated in a homeotropic state that is optically transparent under an electric field.
Conventional CLCs, when in a planar alignment, adopt a spiral arrangement to form a uniform helical structure. They exhibit the property of selectively reflecting light at a pre-set wavelength xcex0=naP with a naturally narrow bandwidth xcex94xcexi=xcex94n P, where xe2x80x9cPxe2x80x9d is the helix pitch, and xcex94n is the CLC birefringence. 50% of the unpolarized incident light within the band is reflected into a circular polarization state that has the same handedness as the CLC spiral, while the remaining 50% is transmitted with the opposite polarization state.
A broadband reflective CLC can be obtained via creating a pitch gradient xcex94P along the to CLC helix. The resulted broad bandwidth is determined by xcex94xcex=n xcex94P. The pitch gradient is created via liquid crystal molecular diffusion under a non-uniform UV field during the polymerization. Both passive and active broadband CLC reflecting films have been obtained. In one of the recent development as disclosed by, xe2x80x9cMethod of Manufacturing a Switchable Cholesteric Filter as well as a Luminaire Having such a Filterxe2x80x9d, R. A. M. Hikmet, PTC, July, 1997, a broadband, switchable CLC film is made from a cholesteric liquid crystal mixture containing a small amount of diacrylate, monoacrylate, non-active nematic liquid crystal, and a non-reactive chiral component. A photo stabilizer is a necessary component in creating such a broadband switchable film. The resulted film operates in the visible spectral region from 448 to 648 nm. No light scattering state has been observed. The switchable broadband CLC has a very high (close to 80%) transmittance when a sufficiently strong field is applied.
In another development, different techniques have been invented that create active broadband polarizers from a cholesteric liquid crystal blend that contains a polymerizable CLC compound and non-polymerizable nematic(s), plus a small amount of photo initiator. Two different polarizers have been developed. In the absence of an electric field, one type of polarizer exhibits a broadband polarizing reflective state in the visible region while the second type polarizer exhibits a narrow band (70 nm) polarizing reflective state in the red spectral region. The first type polarizer can be switched to transmissive state by an AC field; while the second type polarizer is switched from narrow band to broadband reflections by applying a DC voltage.
The broad bandwidth of the first type of polarizer is created by UV induced liquid crystal diffusion mechanism while the bandwidth broadening of the second polarizer is primarily due to the molecular re-orientation of LCs and the subsequent pitch change by the electric field. The first type polarizer is fabricated using a weaker UV light exposure which creates a nonlinear helical pitch distribution along the CLC helical axis. In the absence of an electric field, the reflection band from 440 nm to 660 nm with an average reflectivity around 45% was obtained. Switching of the first type polarizer is realized via vertically re-orienting the CLC molecules by an electric field. Upon applying an AC electric field (10 V/xcexcm), the average reflectivity drops to 2% with a residual reflection peak left near 440 nm which is believed due to the CLC polymer network.
The second type of polarizer is created by a strong UV source (1 W/cm2) and a higher concentration of photo-initiator. As a result, diffusion was restricted during the polymerization, creating a much more uniform helical pitch distribution throughout the mixture and resulting in a spontaneously narrow bandwidth reflective polarizer. In the presence of a DC electric field of 7 V/xcexcm, the polymer network with its own helical structure was not affected due to its high cross-linking density. However, the electric field tends to untwist the non-cross-linked CLC components. Due to the surface constraints and the nature of the cross-linked cholesteric polymer, below the threshold field the non-cross-linked molecules close to the polymer network maintain their orientations, while those further away are aligned along the field. The result is a deformed spiral with a slight mis-orientation that contributes to band broadening.
CLC can also exhibit a focal conic state that scatters light, as shown in FIG. 1. The focal conic state can be generated via different ways. The most conventional method is to electrically trigger a CLC already in a planar state (associated with a naturally narrow band polarizing state) into the focal conic state. In General, such a focal conic state is unstable.
The focal conic state can be stabilized via different methods. The first method involves introduction of a polymer gel network, which is termed as polymer stabilized cholesteric texture (PSCT) as disclosed by, D. K. Yang, L. C. Chien and J. W. Doane, xe2x80x9cCholesteric liquid crystal/polymer gel dispersion for haze-free light shutter,xe2x80x9dAppl. Phys. Lett., 60, 3102-31104 (1992). Two different operating modes can be realized, i.e., the normal mode and reverse moderespectively.
In the absence of an electric field, the normal mode PSCT is in its focal conic state. Applying a sufficient electric field, the CLC molecules are aligned into the homeotropic state which is optically transparent. In this mode, the CLC does not show a planar state during the switching. In a reverse mode PSCT, initially it is in the planar state, characterized by a naturally narrow band reflecting state usually in the IR region. When a suitable electric pulse is applied, the CLC is triggered into a stable focal conic state, which can be triggered back to the original planar state by another electric pulse. In this operating mode, the CLC is switched between the narrow band reflecting state and focal conic state. It does not have a broadband reflection state.
In addition, the switching speed of most prior-art CLC device is relatively slow, in the range of tens or hundreds of milliseconds. There is one prior-art technology in which the switching speed of a surface stabilized cholesteric texture (SSCT) device has been reduced to 150 microseconds typically, or even as low as 10 microseconds, as disclosed by xe2x80x9cUltra Fast Response, Multistable Reflective Cholesteric Liquid Crystal Displaysxe2x80x9d, Bao-Gang Wu, Hongxi Zhou, Yao-Dong Ma, U.S. Pat. No. 5,661,533, 1997, by doping non-ionic surfactants. The function of these surfactants is to isolate the CLC domains from each other while simultaneously reduce the friction forces between the domains and between domains and boundary surface.
Moreover, the surfactant additives produce more uniform distribution of domain size. The net effect of the surfactant additives is to allow continuation of a structure in which the orientation of each microsized liquid crystal domain is stable under zero field, but is much freer to response to a perturbation such as an electric field.
In short, the surfactant functions as a lubricant between the liquid crystal domains and between the domains and the boundaries. The neutral non-ionic surfactant is preferred with a Hydrophile Lipophile Balance (HLB) value ranging from 2.9 to 19. The surfactant can range from 0.1% to 10% of the total weight of the cholesteric liquid crystal mixture. Several surfactants have been found to be effective in reducing the CLC response time. These surfactants are PEG 400 monostearate (HLB=11.6), PEG 1540 monostearate (HLB=17.3), PEG 4000 monostearate (HLB=18.6), Sorbitan monoleate (HLB=15), and Sorbitan monoleate (HLB=10) (all from Chem Service Inc., Westchester, Pa.). Adding a few percent of one of these surfactants into a normal CLC mixture with a pitch in the visible region, the electro-response time has been dramatically reduced from 150 millisecond to 150 microsecond or even 10 microsecond.
Conventional CLC can be light absorptive, in addition to the light scattering, reflection, and transparent states, if a dichroic dye is introduced. Dichroic dye has an elongated molecular shape. It absorbs the light whose electric vibration is along the longer dye axis and passes the light whose electric vibration is perpendicular to the longer axis. The dye can be narrow band absorptive so that it appears to be colorful. Or, the dye can be broadband absorptive so that it looks like a black ink.
Dichroic dye has been added into conventional narrow band CLC and PSCT. In the former case, the dye can have absorption at the same or different wavelength of the CLC reflection peak. In the later case when the PSCT is in the normal mode, the dyed CLC device exhibits a mixed light scattering and absorption state when the CLC is in the focal conic state. When the CLC is switched into the homeotropic state, the device becomes transparent. If the CLC is in reverse mode, in the absence of an electric field, the device exhibits a mixed state of absorption and narrow band reflection. When an electric field is applied or the device is triggered into the focal conic state, the device exhibits a mixed state of light scattering and absorption.
In summary, conventional CLC devices can only achieve the following performance characteristics.
1. An electro-optically-active CLC device that can be switched among a narrow band polarizing state, focal conic light scattering state, and transparent state.
2. An electro-optically-active CLC device that exhibits bi-stable states of a slightly focal conic and distorted planar state. Such a device can be made from either polymer stabilized cholesteric texture (PSCT) or surface stabilized cholesteric texture (SSCT).
3. An electro-optically-active CLC device that can be switched between a focal conic state and a homeotropic state.
4. An electro-optically-active CLC device that can be switched between a broadband reflective polarizing state and a narrow band reflective polarizing state.
5. An electro-optically-active CLC device that can be switched between a broadband reflective polarizing state and a transparent state.
6. An electro-optically-active CLC device that can be switched between a transparent state and a mixed state of light scattering and absorption.
7. An electro-optically-active CLC device that can be switched between a mixed state of light absorption and narrow band reflection and another mixed state of light scattering and light absorption.
However, there does not exist a CLC device that, in a single layer structure, can exhibit the following optical states and can be switched among them: (1) electrically tunable narrow band reflective state, (2) broadband reflective state, (3) light scattering state, (4) transparent state, and, possibly, (5) light absorption state. The technology disclosed in this document can achieve such a high performance liquid crystal and related device(s).
Accordingly, the present invention provides a cholesteric crystal which can be electrically switched and/or tuned to exhibit a single or a combination of the following optical states: (1) light scattering state, (2) transparent state, (3) narrow band circular reflection, (4) broadband circular reflection state, and (5) light absorbing state. Moreover, the single layer devices of the invention are noteworthy in that they are comprised of polymer gels or networks that exhibit multiple optical states, which have been successfully doped with surfactant to achieve improved switching speed.
This technology has been introduced into the currently described liquid crystal material that contains polymer network that exhibits the multiple optical states. Its effectiveness on reducing the response time has been demonstrated. Therefore, it is the second objective of this invention to adopt this technique to reduce the response time of the multi-state cholesteric liquid crystal device.
In one embodiment of the present invention, the device can be electrically switched to one of the following optical states: (1) broadband circular reflection state, (2) light scattering state, (3) narrow band circular reflection, and (4) transparent state.
In another embodiment of the present invention, the device can be switched to one of the following optical states: (1) Broadband reflective state mixed with light scattering state, (2) narrow band reflective, and (3) transparent state.